Ultrasound machines are often used for observing organs in the human body. Typically, these machines contain transducer arrays for converting electrical signals into pressure waves. Generally, the transducer array is in the form of a hand-held probe which may be adjusted in position to direct the ultrasound beam to a region of interest. Transducer arrays may have, for example, 128 transducer elements for generating an ultrasound beam. An electrode is formed on a top and bottom surface of each transducer element and each transducer element is individually excited to generate pressure waves as is well known. The pressure waves generated by the transducer elements are directed toward a region of examination, and more particularly, an object to be observed, such as the heart of a patient being examined. Each time the pressure wave confronts tissue having different acoustic characteristics, a wave is reflected backwards to the transducer. The array of transducer elements converts the reflected pressure waves into corresponding electrical signals. An example of a phased array acoustic imaging system is described in U.S. Pat. No. 4,550,607 which is incorporated herein by reference. That patent illustrates circuitry for combining the incoming signals received by the transducer array to produce a focused image on a display screen.
Broadband transducers are transducers capable of operating at a wide range of frequencies without a loss in sensitivity. As a result of the increased bandwidth provided by broadband transducers, the resolution along the range direction may improve, thereby resulting in better image quality. One possible application for a broadband transducer is contrast or tissue harmonic imaging. In contrast harmonic imaging, contrast agents, such as micro-balloons of protein spheres, are injected into a body to illustrate how much of a certain tissue, such as the heart, is active. These micro-balloons are typically one to five micrometers in diameter and, once injected into the body, may be observed via ultrasound imaging to determine the degree of blood perfusion in the tissue being examined. B. Schrope et al. "Simulated Capillary Blood Flow Measurement Using a Nonlinear Ultrasonic Contrast Agent." Ultrasonic Imaging, Vol. 14 at 134-58 (1992), which is incorporated herein by reference, disclose that an observer may clearly see the contrast agent at the second operating harmonic. That is, at the fundamental frequency, the heart and muscles tissue is clearly visible via ultrasound techniques. However, at the second harmonic, the observer is capable of clearly viewing the contrast agent itself. In addition, it may be desirable to provide an ultrasonic system that is responsive to reflected harmonic signals even when a contrast agent is not injected into a body because tissue itself produces a non-linear response.
Because contrast and tissue harmonic imaging requires that the transducer be capable of operating at a broad range of frequencies (i.e., at both the fundamental and second harmonic), existing transducers typically cannot function at such a broad range. For example, a transducer having a center frequency of 5 Megahertz and having a 70% ratio bandwidth to center frequency has a bandwidth of 3.25 Megahertz to 6.75 Megahertz. If the fundamental frequency is 3.5 Megahertz, then the second harmonic is 7.0 Megahertz. Thus, a transducer having a center frequency of 5 Megahertz would not be able to adequately operate at both the fundamental frequency and second harmonic frequency.
U.S. Pat. Nos. 5,415,175 ("the U.S. Pat. No. '175") and 5,438,998 ("the U.S. Pat. No. '998"), assigned to the present assignee and specifically incorporated herein by reference, disclose a mechanically focused transducer array in which the surface of each transducer element that faces a region of examination when the transducer is in use, is preferably concave in shape and each transducer elements has a non-uniform thickness measured in the range direction. Preferably at least one acoustic matching layer is disposed over each transducer element and the acoustic matching layer also has a non-uniform thickness and a concave surface facing the region of examination. The combination of the curved transducer element and acoustic matching layer provides focusing in the elevation direction. Typically a non-refractive protective layer such as polyurethane is placed on a front surface of the transducer array. Because each transducer element has a non-uniform thickness, by controlling the excitation frequency, an operator can control which section of the transducer element generates the ultrasound beam. At higher excitation frequencies, the beam is primarily generated from the center of the transducer element and at lower excitation frequencies, the beam is primarily generated from the full aperture of the transducer element.
U.S. Pat. No. 4,4788,085 discloses an ultrasound transducer that has a plurality of transducer elements that are non-uniform in thickness. In one embodiment shown in FIGS. 8A-D of this patent, each transducer element has a front surface that faces a region of examination when the transducer is in use that is planar and a back surface, opposite of the front surface, that is concave and each transducer element has a non-uniform thickness measured in the range direction. A single matching layer 32 is disposed over the transducer element 31 and an acoustic lens 33 is disposed on the matching layer. Sasaki does not provide details regarding the acoustic lens material, however, the lens has a convex/concave shape and is probably lossy. In addition to having a transducer which is capable of operating at a broad range of frequencies, two-dimensional transducer arrays are also desirable to increase the resolution of the images produced. An example of a two-dimensional transducer array is illustrated in U.S. Pat. No. 3,833,825 to Haan issued Sep. 3, 1974 and is incorporate herein by reference. Two-dimensional arrays allow for increased control of the excitation of ultrasound beams along the elevation axis, which is otherwise absent from conventional single-dimensional arrays. However, two-dimensional arrays are also difficult to fabricate because they typically require that each element be cut into several segments along the elevation axis, connecting leads for exciting each of the respective segments. A two-dimensional array having 128 elements in the azimuthal axis, for example, would require at least 256 segments, two segments in the elevation direction, as well as interconnecting leads for the segments. In addition, they require rather complicated software in order to excite each of the several segments at appropriate times during the ultrasound scan because there would be at least double the amount of segments which would have to be individually excited as compared with a one-dimensional array.
It is thus desirable to provide a transducer array that employs a low loss acoustic lens and a plurality of sharply focused acoustic matching layers. It is also desirable to provide a transducer array that provide 1.5 or 2 dimensional imaging without the system complexity or channels required of typical 1.5 and 2 dimensional arrays.